Story of new love upset by contents of letter (9)
I believe the answer is:
novelette
'story' is the definition.
(I've seen this before)
'new love upset by contents of letter' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'upset by' indicates an anagram.
'contents of' says to take the centre (I've seen 'content of' mean this).
The central letters of 'letter' are 'ette'.
'love' is an anagram of 'ovel'.
'n'+'ovel'+'ette'='NOVELETTE'
'of' acts as a link.
(Other definitions for novelette that I've seen before include "Short work of fiction" , "Short book of fiction" , "Short story" , "romance?" , "Short and light tale" .)